import crypto from "crypto";

/**
 * Verify POPO webhook signature.
 * Signature = SHA256(token + nonce + timestamp)
 */
export function verifySignature(params: {
  token: string;
  nonce: string;
  timestamp: string;
  signature: string;
}): boolean {
  const { token, nonce, timestamp, signature } = params;
  const data = token + nonce + timestamp;
  const computed = crypto.createHash("sha256").update(data).digest("hex");
  return computed.toLowerCase() === signature.toLowerCase();
}

/**
 * Decrypt POPO encrypted message using AES-CBC.
 * Key derivation: Base64 decode(aesKey + "=") -> 32 bytes, first 16 bytes = IV
 */
export function decryptMessage(encrypt: string, aesKey: string): string {
  // POPO uses Base64(aesKey + "=") as the key source
  const keyBuffer = Buffer.from(aesKey + "=", "base64");
  if (keyBuffer.length < 32) {
    throw new Error("Invalid AES key length");
  }

  // First 16 bytes as IV, full 32 bytes as key
  const iv = keyBuffer.subarray(0, 16);
  const key = keyBuffer.subarray(0, 32);

  // Decrypt the base64-encoded ciphertext
  const ciphertext = Buffer.from(encrypt, "base64");
  const decipher = crypto.createDecipheriv("aes-256-cbc", key, iv);
  let decrypted = decipher.update(ciphertext);
  decrypted = Buffer.concat([decrypted, decipher.final()]);

  // Remove PKCS7 padding and parse as UTF-8
  return decrypted.toString("utf8");
}

/**
 * Encrypt POPO response message using AES-CBC.
 * Used to encrypt the "success" response.
 */
export function encryptMessage(plaintext: string, aesKey: string): string {
  const keyBuffer = Buffer.from(aesKey + "=", "base64");
  if (keyBuffer.length < 32) {
    throw new Error("Invalid AES key length");
  }

  const iv = keyBuffer.subarray(0, 16);
  const key = keyBuffer.subarray(0, 32);

  const cipher = crypto.createCipheriv("aes-256-cbc", key, iv);
  let encrypted = cipher.update(plaintext, "utf8");
  encrypted = Buffer.concat([encrypted, cipher.final()]);

  return encrypted.toString("base64");
}

/**
 * Generate a random nonce for webhook responses.
 */
export function generateNonce(): string {
  return crypto.randomBytes(16).toString("hex");
}